SSDs (Solid State Disks) are known as flash memory devices using flash memories. Flash memory devices are higher in speed than hard disk drives (HDDs), but are more expensive than HDDs. Flash memory devices configured to reduce the amount of data by a compression function for the purpose of reducing bit cost are known.
In a flash memory device configured to store data after compressing every data, however, when compressed data stored in the flash memory device is to be updated, the flash memory device reads the compressed data from a flash memory, expands, updates, and compresses the read data, and writes the resultant data into the flash memory, with the result that transfer performance of the flash memory device is reduced. As a solution, flash memory devices configured to compress a part of data are known.
Patent Literature 1 (PTL 1) indicates that, in reclamation or refresh, a flash memory device determines a read frequency of data to be migrated, compresses data having a read frequency lower than a threshold, and writes the resultant data.